1. Field of the Invention
The present invention relates to a scanning apparatus which forms an image on a scanned surface by a deflected luminous flux, and more particularly relates to a scanning apparatus having a lens system for forming an image by a luminous flux deflected by a deflector at a uniform angular velocity.
2. Description of the Prior Art
A laser scanning apparatus for use in a laser beam printer is known in which after a divergent beam from a laser beam source is converged by a collimator lens, the beam is deflected at a uniform angular velocity by a deflector such as a polygonal mirror, and a scanning lens system forms the beam into a laser beam spot on a scanned surface and scans the scanned surface substantially at a uniform velocity. As the scanning lens systems using an f.theta. lens, many lens systems including a plurality of lenses have been proposed and put into practical use.
Simpler scanning optical systems each using a single scanning lens (single f.theta. lens) are disclosed by Japanese Laid-open Patent Applications Nos. H3-213812 and H4-50908.
The scanning optical systems disclosed by Japanese Laid-open Patent Application No. H3-213812 use a single bi-convex lens or a positive meniscus lens as the f.theta. lens. In the optical systems using the bi-convex lens, coma is restrained in the vicinity of the maximum angle of deflection (angle of deflection is an angle of view between a luminous flux reflected at the reflecting surface and the optical axis. The maximum angle of deflection is the maximum of the angle of view), and field curvature in the main scanning direction and distortion are corrected by providing an aspherical surface for the second surface of the lens. In the optical systems using the positive meniscus lens, field curvature in the main scanning direction and distortion are corrected by using a meniscus lens whose second surface is aspherical and by arranging the meniscus lens so as to be convex to the side of the deflector.
The scanning optical systems disclosed by Japanese Laid-open Patent Application No. H4-50908 use a scanning lens whose both side surfaces are aspherical.
In the scanning optical systems of these laid-open patent applications, a convergent luminous flux is incident on the scanning lens. This is because if a parallel luminous flux is incident, the surface configuration in the main scanning direction must be defined to obtain a refractive power in the main scanning direction, and field curvature in the main scanning direction and distortion cannot sufficiently be corrected.
However, the scanning optical systems of Japanese Laid-open Patent Application No. H3-213812 are defective in performance since distortion is not sufficiently corrected although field curvature in the main scanning direction is sufficiently corrected by providing an aspherical surface for the second surface of the scanning lens. Then, the desired uniform-velocity scanning is not realized. Further, in the optical system, the thickness difference (difference in thickness of the lens between in the vicinity of the maximum angle of deflection and on the optical axis) of the scanning lens is large. Because of the large thickness difference, the following problems arise in molding this scanning lens: the lens surfaces cannot be molded with accuracy; the physical characteristics inside the lens cannot be made uniform; and the amount of shift of the image surface increases which is caused by a change in lens surface configuration or in refractive index due to an increase in temperature or humidity. These problems are remarkable when a resin made scanning lens is used. When a glass made lens is used like in Japanese Laid-open Patent Application No. H3-213812, the cost of the lens increases.
In the scanning optical systems disclosed by Japanese Laid-open Patent Application No. H4-50908, the scanning lens is very difficult to mold since its both side surfaces are aspherical surfaces of very complicated configurations.
Various scanning lenses of a type including one f.theta. lens for forming an image by a luminous flux deflected by a deflector on a scanned surface have also been disclosed by Japanese Laid-open Patent Applications Nos. S62-139520 and S63-157122.
However, according to the conventional arrangements, it is difficult to reduce the size of the whole apparatus while maintaining desired optical performance. The size of the whole apparatus can be reduced by widening the range of the angle of view, i.e. the angle of deflection of the scanning lens to decrease the scanning optical path. However, if the range of the angle of deflection is widen while maintaining the desired optical performance, the distance between the point of deflection and the image surface, and the length of the lens in the main scanning direction increase, thereby increasing the size of the whole apparatus.
For example, in Japanese Laid-open Patent Application No. H3-213812, since the distance from the deflector to the scanning lens is as much as 60 mm, the length of the lens must be increased to widen the range of the angle of deflection. To decrease the length of the lens, the range of the angle of deflection must be narrowed. Thus, the range of the angle of deflection cannot be widened unless the distance from the deflecting surface to the image surface is increased.
In Japanese Laid-open Patent Application No. H4-50908, although the distance from the point of deflection to the scanning lens is 15 mm, the widening of the range of the angle of deflection by decreasing the distance between the scanning lens and the deflector does not directly contribute to the reduction in length of the lens in the main scanning direction. This is because since the curvature of the first surface in the vicinity of the optical axis is convex to the point of deflection, when the angle of deflection increases, the distance between the position at which the luminous flux is incident on the lens surface and the image surface is shorter than the distance between the position of the optical axis on the lens surface and the image surface. In addition, since an aspherical surface of the scanning lens in the main scanning direction is configured to have at least two points of inflection, the mold therefor is difficult to process.
In Japanese Laid-open Patent Applications Nos. S62-139520 and S63-157122, the light beam incident on the deflector is a parallel luminous flux. In the scanning lens using a parallel luminous flux, since aberrations are largely deteriorated when the angle of deflection is large, the angle of deflection cannot be increased, which results in an increase in size of the whole apparatus. In Japanese Laid-open Patent Application No. S62-139520, the scanning lens is of a design ignoring coma. In Japanese Laid-open Patent Application No. S63-157122, although the distance from the deflector to the scanning lens is 10 mm, the thickness of the center of the lens is as much as 30 mm to 60 mm. Further, although the angle of deflection is increased, the increase in angle of deflection makes substantially no contribution to the reduction in size of the apparatus since the distance from the scanning lens to the image surface is as much as 170 mm.